Tissue engineering provides a promising approach to repair tissue and organ defects. Typically, cells are cultured into a 3D biocompatible and/or biodegradable scaffold followed by in vivo implantation. Since the scaffold acts as an artificial extracellular matrix (ECM), it would be desired to mimic critical features of natural ECMs. From the structural perspective, a natural ECM consists of interwoven protein fibers with diameters in the range from tens to hundreds of nanometers. For example, collagen, which forms fiber bundles with diameters of 50-500 nm, is the main component of natural ECMs for many tissues (e.g., skin, bone, and tendon). The nanofibrous structure of ECMs offers a network to support cells and to present an instructive background to guide cell behaviors.
The development of scaffolds that possess similar morphological structures of natural ECMs is one major challenge in tissue engineering. Presently, there are generally three methods to prepare nanofibrous scaffolds including self-assembly, phase separation, and electrospinning; however, all of the three methods have limitations. For example, the self-assembly method is difficult to control the pore size/shape inside a scaffold; and most self-assembled scaffolds are prepared in liquid environment, resulting in relatively weak mechanical properties. The phase separation method has little control over the fiber diameter/orientation in a scaffold, and the preparation time is typically very long; furthermore, such a method can only generate small pores with sizes up to about 10 μm. Although the phase separation method can be combined with the porogen leaching technique to make the needed macropores (with sizes from tens to hundreds of micrometers), such an approach is usually time-consuming and often the complete removal of porogen from a resulting scaffold is difficult.
The electrospinning method has attracted growing attentions due to its capability to make nanofibers similar to the fibrous structures in natural ECMs, and the method can be applied to a wide range of materials. Small diameters and the concomitant large surface area of electrospun nanofibers, as well as the porous structures of electrospun nanofibrous mats, can facilitate cell adhesion, proliferation, migration, and differentiation. These advantageous features make electrospun nanofibrous scaffolds well-suited for tissue engineering. However, the major limitation of electrospun scaffolds is owing to their morphological structure of overlaid nanofiber mats with apparent pore sizes in sub-micrometers; i.e., as-electrospun nanofibrous mats lack the needed macropores (with sizes from tens to hundreds of micrometers) for cell growth. Hence, it is important to develop an innovative strategy to fabricate electrospun 3D nanofibrous scaffolds with interconnected and hierarchically structured pores and high porosities; such scaffolds would better mimic natural ECMs, thereby maximizing the likelihood of long-term cell survival and the resulting generation of functional tissue in a biomimetic environment.
Several approaches have been explored recently for the fabrication of electrospun 3D nanofibrous scaffolds such as multi-layering electrospinning, electrospinning followed by various post-treatments, liquid-assisted collection, template-assisted collection, porogen-added electrospinning, and self-assembly. These approaches have proven insufficient as the scaffolds still lack desired pore structures (including size, shape, and interconnectivity of pores, as well as overall porosity of structures/scaffolds), structural flexibility, and mechanical properties.
Thus, electrospun 3D nanofibrous scaffolds and their preparation methods are still desired. It is desired that the scaffolds: (1) be highly porous to allow for cell growth and mass transport; (2) have hierarchically structured pores, so that the large pores can allow for cell penetration and the small pores can allow for mass transport; (3) have fibers with diameters in a range from tens to hundreds of nanometers; (4) possess stable shape with good mechanical properties, in particular, properties suitable for immersion in cell culture media; and (5) have the capability to form any desired shape.
Accordingly, it is an object of the invention to provide three-dimensional nanofibrous scaffolds suitable for tissue engineering and methods of preparing the same.